Electric assist steering systems are well known in the art. Electric power assist steering systems that utilize a rack and pinion gear set provide power assist by using an electric motor to either (i) apply rotary force to a steering shaft connected to a pinion gear, or (ii) apply linear force to a steering member having the rack teeth thereon. The electric motor in such systems is typically controlled in response to (i) a driver's applied torque to the vehicle steering wheel, and (ii) sensed vehicle speed.
U.S. Pat. No. 4,415,054 to Drutchas (now U.S. Reissue Pat. No. 32,222,), assigned to TRW Inc., utilizes a D.C. electric assist motor driven through an "H-bridge" arrangement. The assist motor includes a rotor encircling a steering member. The steering member has a first portion with a thread convolution and a second portion with straight cut rack teeth. Rotation of the electric assist motor rotor causes linear movement of the steering member through a ball-nut drivably connected to the thread convolution portion of the steering member. A torque sensing device is coupled to the steering wheel for sensing driver applied torque to the steering wheel. The torque sensing device uses a magnetic Hall-effect sensor that senses relative rotation between the input and output steering shafts across a torsion bar. An electronic control unit ("ECU") monitors the signal from the torque sensing device. A vehicle speed sensor provides a signal to the ECU indicative of the vehicle speed. The ECU controls current through the electric assist motor and, in turn, steering assist in response to both the sensed vehicle speed and the sensed applied steering torque. The ECU decreases steering assist as vehicle speed increases. This is commonly referred to in the art as speed proportional steering.
U.S. Pat. No. 5,257,828 to Miller et al., and assigned to TRW Inc., discloses an electric assist steering system having yaw rate control. This system uses a variable reluctance ("VR") motor to apply steering assist to the rack member. The torque demand signal is modified as a function of a steering rate feedback signal so as to provide damping.
U.S. Pat. No. 5,504,403 to McLaughlin, and assigned to TRW Inc., discloses a method and apparatus for controlling an electric assist steering system using an adaptive blending torque filter. The adaptive blending torque filter processes the applied steering torque signal and maintains a selectable system bandwidth during system operation. This arrangement provides a steering system having a bandwidth that is substantially independent of vehicle speed and applied steering torque.
Ideally, the electric motor of an electric assist steering system will have a bandwidth much greater than that of the electric steering system so that the response of the electric motor does not negatively impact the stability of the steering system. A variable reluctance motor is such a high bandwidth motor. A constant bandwidth motor is desirable so as to achieve control not only over the low frequency steering operation, but also over the higher frequency acoustic noise so that the motor is quiet. However, the uncontrolled bandwidth of a VR motor varies and is a function of the motor current i, the rotor position .theta. relative to the stator, the motor resistance, and motor temperature t. It is desirable to maintain a consistent system bandwidth independent of such motor operating conditions. The controller must compensate for this varying bandwidth to achieve a constant bandwidth. VR motors have acoustically sensitive structural modes in which the motor's stator housing ("shell") experience movement in a radial direction, and at particular drive frequencies, the motor shell will resonate. Unfortunately, this resonance can occur in the human audible range. The motor will, in effect, act as a "speaker" producing an undesirable motor buzz. The motor can further exhibit a "microphone" effect as a result of shell acceleration resulting in current oscillations in the motor coils inducing further noise out of the motor. It is, therefore, desirable to reduce such acoustic noise and, in turn, torque ripple.